Photovoltaic power generation systems are now being commercially introduced and coming into widespread use. A solar cell utilizing a semiconductor junction of silicon, gallium arsenide or the like is generally known as a method of converting light energy into electric energy. A crystal silicon solar cell, a polycrystalline silicon solar cell utilizing a p-n junction of a semiconductor, and an amorphous silicon solar cell utilizing a p-i-n junction of a semiconductor have all been developed for practical applications. However, the production cost of silicon solar cells is relatively high and much energy is consumed in the process. Thus, in order for these solar cells to be cost effective it becomes necessary to use them for a long duration. These high costs prevent the wide use of silicon solar cells today. Solar cells using CdTe and CuIn(Ga)Se have also been studied. However, environmental pollution and resource consumption are serious constraints in their large-scale deployment.
In addition to those dry type solar cells, a wet type solar cell utilizing a photoelectric chemical reaction caused in the interface of a semiconductor and an electrolytic solution has also been studied. A metal oxide semiconductor such as titanium oxide, tin oxide, or the like has the advantage of lowering solar-cell manufacturing costs as compared to silicon, gallium arsenide, or the like. Above all, titanium oxide and zinc oxide are excellent in both photoelectric conversion efficiency in the ultraviolet region and photochemical stability. But, when used alone, semiconductors such as titanium oxide and zinc oxide (having a wide band gap not less than 3 eV) absorb poorly outside the ultraviolet range of the spectrum and therefore the photoelectric conversion is inefficient.
The cost of biologically-derived light harvesting components has commercial potential because of the wide availability and ease of directed growth of plants, seaweed, algae and bacteria including thermophilic organisms.
The first truly bio sensitized photovoltaic cell was reported by Das et al. (Nanoletters, 2004, 1079-1083) using photosystem I (PS-I) or the bacterial reaction center (RC), but with very low efficiencies and lifetimes. Das et al. (incorporated herein by reference) described the photovoltaic devices using a self-assembled monolayer of Ni2+-NTA on transparent and conductive indium-tin oxide (ITO)-coated glass. The photosynthetic components are oriented via selective binding with polyhistidine tags present on each complex. However, the device suffers from a very low efficiency.
Notwithstanding the recent advances in photoelectronic devices, the need for more efficient photovoltaic cells at a lower cost of production still exists.